The CTIA (Capacitor Transimpedance Amplifier) is utilized in infrared and other sensing applications to integrate the current from a detector for a specified period of time, referred to as the integration time. Referring to FIG. 1, an exemplary CTIA 3 of a particular unit cell contains a high gain inverting amplifier having a driver 2 with a capacitor in the feedback loop (C.sub.FB). The inverting amplifier typically contains, as a minimum, two active transistors or MOSFETs. A first transistor or MOSFET is used to provide a constant current source (typically referred to as the load 1), while the second transistor or MOSFET is used to implement the driver 2. A reset switch (typically another transistor) is placed across the feedback capacitor and is closed to discharge the capacitor and is then opened to begin the integration time. The output voltage of the CTIA 3 is proportional to the product of the detector current (I.sub.D) and the integration time, and is inversely proportional to the value of the feedback capacitor C.sub.FB. The input voltage is maintained near the reset value by the feedback loop, which maintains a nearly constant bias on the radiation detector. At the end of the integration time the output voltage is sampled by closing an output multiplexer (MUX) switch, the reset switch is then closed, and the CTIA 3 is ready for the next integration.
FIG. 1 shows a conventional case where a two dimensional array of detectors and unit cells are arranged in a row and column (x by y) matrix (only a part of one column is depicted). Typically the MUX switches are closed and then opened one after another to readout in sequence the x unit cell outputs from each of the rows connected to a single one of the y column output lines. Also connected to the column line may be an input of a sample and hold (S/H) circuit (not shown), followed by a voltage follower (not shown). The output voltages may eventually be converted to a digital form and then operated on by a data processing system for performing any desired image processing, or to simply store the image(s) for subsequent transmission to another location. This latter type of operation is typical in space-based and other types of astronomy applications.
One drawback to the use of the conventional CTIA is that it is an active amplifier that requires continuous current. This current is a dominant source of power dissipation in the conventional CTIA, and is also a source of light emission. That is, it is known that, when powered on and operating, silicon-based MOSFET circuits will generate a small amount of IR radiation (typically in the one micron range). Both of these effects of normal operation (i.e., power dissipation and IR light generation) are disadvantageous, especially so when the CTIA is used in a low temperature system with limited cooling capacity, and/or in those systems intended to detect low light levels.
For example, in some astronomy applications, such as deep field galaxy surveys, one may be imaging distant objects over a period of hours or even days, literally on a photon-by-photon basis. As may be appreciated, in such low light level applications it is important to reduce or eliminate any extraneous sources of detectable energy which may deteriorate the signal to noise ratio of the imaging system.
A further drawback to the use of the conventional CTIA is the complexity of the amplifier at each detector element, commonly referred to as the unit cell. That is, since each CTIA 3 of each unit cell has its own associated load 1 (which can be a resistance but is more typically implemented as a transistor (e.g., a MOSFET) connected so as to form the constant current source), the circuit area required to lay out the unit cell is increased, and the overall yield in large arrays is thus also reduced. Reference in this regard can be had to, by example, FIG. 6 of U.S. Pat. No.: 4,978,872, "Integrating Capacitively Coupled Transimpedance Amplifier", by Morse et al., where a MOSFET load 122 is shown.
Further reference with regard to various aspects of CTIAs may be had to the following U.S. Patents, namely U.S. Pat. No.: 4,956,716, "Imaging System Employing Charge Amplifier", by Hewitt et al.; U.S. Pat. No.: 5,043,820, "Focal Plane Array Readout Employing One Capacitive Feedback Transimpedance Amplifier For Each Column", by Wyles et al.; U.S. Pat. No.: 5,602,511, "Capacitive Transimpedance Amplifier Having Dynamic Compression", by Woolaway; and U.S. Pat. No. 4,786,831, entitled "Integrating Capacitively Coupled Transimpedance Amplifier", by Morse et al. The disclosures of these U.S. Patents are incorporated by reference herein in their entireties.